A client who is ignorant but not arrogant doesn’t know anything about advertising, cheerfully admits it, and defers to your superior knowledge and expertise.
A client who is arrogant but not ignorant is one who knows at least as much about advertising as you do, and more than you do about their product and market. The best relationship with this client is collaborative in nature.
A client who is both arrogant and ignorant is the one who cheerfully admits she knows nothing about advertising, and then proceeds to tell the copywriter or ad agency how to do their job. If there is a disagreement, she seldom listens to advice, and trusts her own admittedly minimal instincts and experience instead.
The client who is arrogent and ignorant should be avoided. You may tell her there should be only one cook in the kitchen (you), or that if she hired you as the chauffer she should let you drive, but it is usually to no avail. It is nearly impossible to do good work for the simultaenously ignorant and arrogent client.
Which kinds of clients do you serve? Ignorant? Arrogant? Neither? Both? And what is your reaction when they make changes that in your opinion will make the marketing less effective?